On Oct 17, 6:42*am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the circuit..

For a screwdriver antenna, the problem is solved by a conductive
sleeve over the outside of the shorted turns that keeps most of the RF
on the conductive sleeve instead of in the shorted turns of the coil.
--
73, Cecil, w5dxp.com

On Oct 17, 2:36*pm, Cecil Moore wrote:
On Oct 17, 6:42*am, Alejandro Lieber alejan...@Use-Author-Supplied-

Address.invalid wrote:
It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the circuit.

For a screwdriver antenna, the problem is solved by a conductive
sleeve over the outside of the shorted turns that keeps most of the RF
on the conductive sleeve instead of in the shorted turns of the coil.
--
73, Cecil, w5dxp.com

that doesn't really 'solve' it, that just provides a single big
shorted turn instead of many turns.

On Oct 17, 9:52*am, K1TTT wrote:
that doesn't really 'solve' it, that just provides a single big
shorted turn instead of many turns.

Maybe the single big shorted turn IS the solution? :-) When there are
no actual shorted turns, i.e. all of the coil is exposed, does the
bottom section of aluminum tubing become that same single big shorted
turn? In any case, one very conductive and very wide shorted turn is a
lot less lossy than a number of small shorted turns.
--
73, Cecil, w5dxp.com

On Oct 17, 3:09*pm, Cecil Moore wrote:
On Oct 17, 9:52*am, K1TTT wrote:

that doesn't really 'solve' it, that just provides a single big
shorted turn instead of many turns.

Maybe the single big shorted turn IS the solution? :-) When there are
no actual shorted turns, i.e. all of the coil is exposed, does the
bottom section of aluminum tubing become that same single big shorted
turn?

no, it becomes a very small diameter and long shorted turn... but yes,
it is a shorted turn. you don't see as much effect because it is
smaller in diameter so the self inductance is smaller and it only
intercepts a fraction of the flux from the end of the coil. you get
more of an effect if you place an air core coil with its end near the
side or bottom of a metallic enclosure, the plate looks like a shorted
coil and can have large circulating currents.

In any case, one very conductive and very wide shorted turn is a
lot less lossy than a number of small shorted turns.
--
73, Cecil, w5dxp.com

that is why they get away with it, the losses in the big fat thing are
low enough that it causes less trouble than shorting turns with a
switch or relays.

Cecil Moore wrote:
On Oct 17, 6:42 am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the circuit.

For a screwdriver antenna, the problem is solved by a conductive
sleeve over the outside of the shorted turns that keeps most of the RF
on the conductive sleeve instead of in the shorted turns of the coil.
--
73, Cecil, w5dxp.com

But, isn't that conductive sleeve itself a shorted turn? It's
conductive, coaxial with the rest of the inductor above the sleeve, so
the magnetic field certainly passes through it.

I think the real answer is that everything is a tradeoff.

On Oct 18, 11:04*am, Jim Lux wrote:
But, isn't that conductive sleeve itself a shorted turn?

Yes, but that particular low-loss shorted turn solves the problem that
needs solving. Nobody said it was a perfect solution.
--
73, Cecil, w5dxp.com

On Oct 18, 9:57*pm, Cecil Moore wrote:
On Oct 18, 11:04*am, Jim Lux wrote:

But, isn't that conductive sleeve itself a shorted turn?

Yes, but that particular low-loss shorted turn solves the problem that
needs solving. Nobody said it was a perfect solution.
--
73, Cecil, w5dxp.com

well, if its not the perfect solution then the problem is not
completely solved... so there must be a better solution to really
solve the problem.

On Oct 18, 5:13*pm, K1TTT wrote:
well, if its not the perfect solution then the problem is not
completely solved... so there must be a better solution to really
solve the problem.

The problem of transmission line losses can be solved with a perfect
lossless transmission line. Have you seen such or does reality force
us to settle for a reasonable non-perfect solution?
--
73, Cecil, w5dxp.com

K1TTT wrote:
On Oct 18, 9:57 pm, Cecil Moore wrote:
On Oct 18, 11:04 am, Jim Lux wrote:

But, isn't that conductive sleeve itself a shorted turn?
Yes, but that particular low-loss shorted turn solves the problem that
needs solving. Nobody said it was a perfect solution.
--
73, Cecil, w5dxp.com

well, if its not the perfect solution then the problem is not
completely solved... so there must be a better solution to really
solve the problem.

one could simply slit the tube (and finger stock at top). Consider that
the lower tube is serving two purposes: mechanical support and a movable
contact on the inductor.

On 10/18/10 12:04 PM, Jim Lux wrote:
Cecil Moore wrote:
On Oct 17, 6:42 am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the
circuit.

For a screwdriver antenna, the problem is solved by a conductive
sleeve over the outside of the shorted turns that keeps most of the RF
on the conductive sleeve instead of in the shorted turns of the coil.
--
73, Cecil, w5dxp.com

But, isn't that conductive sleeve itself a shorted turn? It's
conductive, coaxial with the rest of the inductor above the sleeve, so
the magnetic field certainly passes through it.

Whic of course means that everything everywhere is a shorted turn. 8^)


I think the real answer is that everything is a tradeoff.

That's a pretty good answer.